Process for the treatment of gases

ABSTRACT

An installation and a method are described for the treatment of a gas containing hydrogen sulphide, the gas (1) being washed in a first gas scrubber (5) with an alkaline wash liquid and the spent wash liquid (4) being treated in an aerobic reactor (6) with oxygen in the presence of sulphide-oxidising bacteria and the effluent (9) from the aerobic reactor (6) being re-used as wash liquid (3) and the elementary sulphur formed during the treatment with oxygen being removed from the effluent (19), the effluent (13) from which sulphur has been removed being treated in an anaerobic reactor (17) with sulphate-reducing bacteria and returned to the aerobic reactor (6). The installation and the method can also be used for simultaneous removal of SO 2 , COS, CS 2 , NH 3  and HCN.

The invention relates to a process for the treatment of a gas containinghydrogen sulphide and optionally other pollutants, the gas being washedin a first gas scrubber with an alkaline wash liquid and the spent washliquid being treated in a first aerobic reactor with oxygen in thepresence of sulphide-oxidising bacteria and the effluent from the firstaerobic reactor being re-used as wash liquid and elementary sulphurformed during the treatment with oxygen being removed from the effluent.

A process of this type is disclosed in International Patent ApplicationWO 92/10270. This process is suitable for the removal of hydrogensulphide (H₂ S) and optionally other reduced sulphur compounds, such asmercaptans and carbon disulphide, or for the removal of sulphur dioxide(SO₂).

A disadvantage of the known method is that small amounts of sulphate areproduced during the biological oxidation of sulphide and that nosolution is provided for preventing the undesired accumulation thereof.The known method is also not suitable for the removal of otherpollutants which can be present in addition to H₂ S, such as ammonia(NH₃), hydrocyanic acid (HCN), sulphur dioxide (SO₂), carbonyl sulphide(COS) and/or carbon disulphide (CS₂).

A process has now been found for the treatment of gases, which allowsremoval of hydrogen sulphide without appreciable residues and which alsoallows other undesirable gaseous components frequently encountered, suchas ammonia, hydrocyanic acid, sulphur dioxide, carbon disulphide orcarbonyl sulphide, to be removed without separate pretreatment orpost-treatments and associated installations being required for this.The process produces only solid elementary sulphur and, if the gas to betreated also contains nitrogen compounds such as NH₃ or HCN, molecularnitrogen (N₂), both of which can be used or discharged without anydrawbacks. The process is particularly suitable for the treatment offuel gases (natural gas, coal gas) and other gases which are usefullyused after treatment. The process is also suitable for the treatment ofgases which will no longer be used and ultimately will be discharged,optionally after burning off, such as flue gases and industrial gases,for example Claus off-gases (gases which are produced during thereaction of high concentrations of H₂ S with SO₂ with the formation ofelementary sulphur).

The process according to the invention is characterised in that theeffluent from the first aerobic reactor, from which sulphur has beenseparated off, is treated in an anaerobic reactor with sulphate-reducingbacteria and returned to the first aerobic reactor.

As a result of the use of the anaerobic reactor connected downstream ofthe aerobic reactor, the sulphate which leaves the aerobic reactor isreduced to sulphide. When the gas to be treated contains sulphur dioxidein addition to hydrogen sulphide, this sulphur dioxide (in the form ofsulphite or sulphate) is reduced to sulphide as well.

The bacteria which are active for reduction of sulphate and sulphite andother oxidised sulphur compounds in the anaerobic reactor (designatedhere as sulphate-reducing bacteria) are, for example, bacteria of thegenera Desulfovibrio, Desulfotomaculum, Desulfomonas, Desulfobulbus,Desulfobacter, Desulfococcus, Desulfonema, Desulfosarcina,Desulfobacterium and Desulfuromonas. Bacteria of this type are availablewithout any problem from diverse anaerobic cultures and/or growspontaneously in the anaerobic reactor.

Reduction equivalents (electron donors) are needed for the biologicalreduction in the anaerobic reactor. Suitable electron donors are, interalia, hydrogen, carbon monoxide, lower alcohols (for example methanoland ethanol) and other organic substances which can easily be oxidisedby biological means, such as acetate, propionate, glucose, sucrose,starch and the like.

The sulphide-containing effluent from the anaerobic reactor is recycledto the aerobic reactor, where the sulphide is again largely convertedinto elementary sulphur.

The amount of oxygen fed to the aerobic reactor is regulated such thatit is in the main elementary sulphur which is produced on oxidation ofthe absorbed sulphide.

Suitable bacteria which oxidise sulphide and other sulphur compoundshaving a low degree of oxidation to elementary sulphur in the aerobicreactor in the presence of oxygen (designated here as sulphide-oxidisingbacteria) are the autotrophic aerobic cultures known for this purpose,such as the autotrophic aerobic cultures of the genera Thiobacillus andThiomicrospira.

The formation of sulphur in the aerobic reactor leads to a sulphursuspension, which is tapped off. The sulphur from this suspension isseparated off and worked up by drying and, optionally, purifying, andre-used.

Approximately 90% of the alkalinity used in the gas scrubber isre-formed during the oxidation in the aerobic reactor. The bulk of theother 10% of the alkalinity is re-formed in the anaerobic reactor.

A portion of the effluent from the aerobic reactor is recycled to thegas scrubber as wash liquid. Preferably, said effluent has a pH of7.5-9.5, in particular of 8-9.2. A relatively high pH, such as between 9and 9.5, has the advantage that bicarbonate (CO₂) stays better insolution and the buffer action is thus more effective. Elementarysulphur is more stable at a lower pH of, for example, 8-8.5.

If necessary, the pH is adjusted by adding alkali or sodium carbonate.If the gas to be treated contains little CO₂, as in the case of naturalgas, (<5% CO₂), CO₂ or an equivalent thereof, such as (bi)carbonate, ispreferably added, both to adjust the pH and to increase the bufferaction. This addition can be made in the aerobic reactor so that the pHis adjusted to the desired value in this reactor, or can be made in the(first) gas scrubber. An organic electron donor, such as acetate,sugars, and the like, can also be introduced into the anaerobic reactor,where it is converted into (bi)carbonate by the bacteria present in saidreactor. In this way the electron-donating, pH-raising and bufferfunctions are combined.

In general, neutralising agents are not needed to lower the pHdownstream of the scrubber and, therefore, hardly any salts build up inthe recirculating wash liquid.

Because the effluent from the aerobic reactor, from which the elementarysulphur has not yet been separated off or has not yet been completelyseparated off, is preferably used as wash liquid, the wash watercontains elementary sulphur which promotes the absorption of H₂ S, butalso of SO₂ and HCN, from the gases to be treated. This leads to theformation of, respectively, disulphide and polysulphide (HS_(n) ⁻ ;n≧2),thiosulphate (HS₂ O₃ ⁻) and thiocyanate (SCN⁻). Preferably, the washliquid contains 1-50, in particular 10-50 g elementary sulphur per 1.

Elementary sulphur in the wash liquid is useful especially in the casewhere HCN is present as a component in the gas. The cyanide, which istoxic to the majority of bacteria, is converted by the elementarysulphur into the far less toxic thiocyanate, which is then broken downbiologically and/or chemically as can be seen from the followingreaction equations: ##STR1##

Thus, HCN is ultimately converted to carbon dioxide and ammonia. Theammonia can be further reacted, if desired, as explained below.

When the gas to be treated contains other volatile sulphur compounds,such as lower alkyl mercaptans or carbon disulphide, in addition to H₂S, the spent wash liquid which contains the sulphur compounds can be feddirectly into the aerobic reactor containing the sulphide-oxidisingbacteria. When the reduced sulphur compounds have dissolved, they aretermed "sulphide", but this term is also understood to include otherreduced sulphur compounds, such as dissolved hydrogen sulphide (H₂ S orHS⁻), disulphide, polysulphide, thiocarbonates, alkanethiolates, and thelike.

If the gas also contains CO₂, the latter will also be partially absorbedin the wash liquid. The absorbed carbon dioxide will, in the form ofbicarbonate, have a favourable buffer action on the wash liquid.Moreover, some of the CO₂ will be stripped in the aerobic reactor, whichleads to a rise in pH.

The sulphide concentration in the spent wash liquid, which has a pH ofabout 8.5, will usually be, expressed as sulphur, approximately 80-800mg/l when treating gases under approximately atmospheric pressure. Thisis a lower concentration than the concentration reached in aconventional H₂ S scrubber operating at a pH of 10 to 11. The scrubberwill therefore have to be larger than a conventional scrubber and ahigher water/gas stream ratio will be used, for example a ratio of waterstream to gas stream of 0.1 to 0.25. In the case of compressed gases,such as fuel gas (under, for example, 20 bar) or natural gas (under, forexample, 75 bar), the sulphide concentration can rise to 3 g/l and therequirements in respect of the scrubber and the water/gas ratio are thusless stringent.

The process according to the invention has advantages in particular forthe treatment of gases which contain appreciable amounts of otherundesirable components. An important additional pollutant is ammonia.When gas has to be treated which in addition to H₂ S (and possibly SO₂)also contains ammonia, this will to a certain extent be absorbed in thegas scrubber discussed and reacted in the aerobic reactor.

However, it is preferable, if it is desired drastically to restrict thequantity of ammonia to be discharged, to use a second gas scrubber,connected upstream of the first, and to wash the gas in said secondscrubber using a wash liquid of a lower pH. In this way NH₃ is washedout more efficiently. With this arrangement, the wash liquid in thesecond gas scrubber preferably has a pH of 5 to 7.5 and in particular of6-7. Preferably, the gas is first washed in the second gas scrubberwhich has the lower pH, mainly with a view to absorption of NH₃, and isthen washed in the gas scrubber described first which has the higher pH,especially with a view to absorption of H₂ S.

The spent wash liquid from the second gas scrubber can then be combinedwith the spent wash liquid from the first gas scrubber and subjectedsuccessively to biological oxidation and reduction. A large proportionof the ammonia will not be converted in the aerobic reactor wheresulphide is oxidised. That is why a second aerobic reactor is preferablyused, said second reactor being fed with the effluent from the firstaerobic reactor, from which elementary sulphur has been separated off asfar as possible.

In said second aerobic reactor, ammonia is converted to nitrate bynitrifying bacteria in the presence of oxygen. A portion of the effluentfrom the nitrifying aerobic reactor is then fed into the anaerobicreactor which has already been discussed, where the nitrate is reducedto nitrogen by denitrifying bacteria in the presence of an electrondonor, such as hydrogen. The effluent from the anaerobic reactor isrecycled to the first aerobic reactor, as already described above. Thefollowing reactions occur:

(1) NH₄ ⁺ +2O₂ →NO₃ ⁻ +2H⁺ +H₂ O second aerobic reactor

(2) 2NO₃ ⁻ +5H₂ →N₂ +4H₂ O +2OH⁻ anaerobic reactor

(3) 2NO₃ ⁻ +5HS⁻ +H₂ O →N₂ +5S⁰ +7OH⁻ first aerobic reactor

As the pH of the effluent from the second aerobic reactor has beenlowered as a result of nitrate formation, a portion of this effluent isadvantageously used as wash liquid for the second gas scrubber. Ifnecessary, the pH can be further adjusted by adding acid or base or bymixing with other effluents. The nitrate can then be partially convertedto molecular nitrogen (reaction (3)) in the first aerobic reactor.

The process according to the invention is also outstandingly suitablefor use for the treatment of gas which also contains hydrocyanic acid inaddition to H₂ S (and possibly SO₂ and/or NH₃). As already described,the absorption of the HCN can be promoted by means of elementary sulphurdissolved or suspended in the wash liquid. In the case of two gasscrubbers as described above, the HCN will be absorbed mainly in thefirst, more alkaline gas scrubber (connected downstream of the secondgas scrubber). The nitrate formed is ultimately converted to nitrogen bythe combination of anaerobic and aerobic reactors.

If the gas to be treated contains carbon disulphide and/or carbonylsulphide, the latter can likewise be effectively removed using theprocess according to the invention. In this case more than one gasscrubber is preferably used, as has been described above for the removalof ammonia. For very thorough removal of COS, two gas scrubbers of thealkaline type are used in series, it being possible to use effluent fromthe first aerobic reactor as wash liquid for both scrubbers. AbsorbedCOS and CS₂ is essentially converted into carbonate and sulphur in theaerobic reactor.

The gas scrubbers to be used according to the invention can be of aconventional type, provided effective contact between the gas stream andthe wash liquid is produced in the gas scrubbers.

The anaerobic and aerobic reactors to be used according to the inventioncan be of any suitable type. Reactors of the vertical circulating type,such as are described, for example, in International Patent Application94/29227, in which the gas to be used (this is usually air in theaerobic reactor) can provide for vertical circulation, are preferablyused, in particular for the aerobic reactor(s).

The invention also relates to an installation for carrying out theprocess as described above.

In the accompanying drawings:

FIG. 1 is a flow diagram of an installation for carrying out the processaccording to the present invention; and

FIG. 2 is a view similar to FIG. 1 but showing a modified embodimentthereof.

EXAMPLES AND DESCRIPTION OF THE FIGURES EXAMPLE 1 Removal of sulphurcompounds in the absence of significant amounts of nitrogen compounds

Claus off-gas (approx. 40% CO₂, 45% H₂ O,. 8% N₂) containing H₂ S andSO₂ as the main pollutants and additionally containing COS and CS₂ istreated in an installation as shown in FIG. 1.

The polluted gas enters gas scrubber 5 at the bottom via line 1. Theclean gas leaves the scrubber at the top via line 2. The gas is treatedusing wash liquid which is supplied via 3. The wash liquid laden withsulphide and, possibly, sulphite leaves the scrubber at the bottom via 4and is fed to the aerobic reactor 6. Nutrients for the biomass can beadded, if necessary, at 4.

In the aerobic reactor 6, sulphide in the wash liquid is convertedmainly to sulphur by means of bacteria and oxygen. The reactor issupplied with air via aeration system 7. The spent air can usually bereleased via 8 into the outside air without any problem.

Some of the effluent from the aerobic reactor 6 is used as wash liquidfor the gas scrubber 5. The remainder is fed via 10 to the sulphurseparator 11, in which the bulk of the sulphur is removed therefrom. Thesulphur slurry separated off is removed via 12, dehydrated and, ifnecessary, purified for re-use.

Stream 13, which contains sulphate/sulphite, is fed to the anaerobicreactor 17. An electron donor is added via 15. In the case of a gaseouselectron donor, a discharge gas is released via 16 and optionallypartially recycled. In the anaerobic reactor sulphate and sulphite areconverted to sulphide. The effluent is recycled via 14 to the aerobicreactor 6 where the sulphide is again converted mainly to sulphur. Asmall discharge 18 is necessary to prevent accumulation of pollutant,non-degradable components. If the discharge is located downstream of theanaerobic reactor, as in FIG. 1, this will contain sulphide and usuallyhas to be subjected to an after-treatment. If the discharge of lowconcentrations of sulphate is acceptable, the discharge can be locatedat the level of line 13 and this does not require any after-treatment.

In the case of Claus off-gas described here, the polluted gas alsocontains COS and/or CS₂ and the treated gas is no longer used (it isusually burnt off). The gas is then subjected to an after-treatmentdownstream of 5 in a biological trickling filter installation 19. Tothis end, air is supplied via 21 to filter installation 19, whichcontains a biomass similar to that in the aerobic reactor 6. COS and CS₂residues are absorbed here and converted essentially into sulphur and/orsulphate, and CO₂. The treated gas is discharged via 20. The wash waterfrom the bio-scrubber originates from the aerobic reactor via 22 and isreturned to the aerobic reactor via 23. The results of the treatmentaccording to this example are shown in the table below.

                  TABLE                                                           ______________________________________                                        Stream         Example 1 Example 2 Example 3                                  ______________________________________                                        1      flow rate   5000 Nm.sup.3 /h                                                                        13500 m.sup.3 /h*                                                                     1500 m.sup.3 /h*                            H.sub.2 S (vol. %) 0.4 0.3 0.3                                                SO.sub.2 (vol. %) 0.2 --  --                                                  COS (ppm) 500 -- 400                                                          CS.sub.2 (ppm) 280 --                                                         NH.sub.3 (ppm) --  -- 150                                                     HCN (ppm) -- -- 150                                                          2 H.sub.2 S (ppm) <5 <5                                                        SO.sub.2 (ppm) <5 --                                                         20 H.sub.2 S (ppm) 0 n/a <5                                                    COS (ppm) 125 n/a 100                                                         CS.sub.2 (ppm) 140 n/a                                                        NH.sub.3 (ppm) --  n/a <5                                                     HCN (ppm) -- n/a <5                                                          33/34 flow rate (m.sup.3 /h) n/a n/a 85                                       3/4 flow rate (m.sup.3 /h) 85 1450 1530                                       22/23 flow rate (m.sup.3 /h) 20 n/a 85                                        12 10% S slurry 0.4 10 10                                                     13 flow rate (m.sup.3 /h) 25 150                                               SO.sub.4.sup.2-  (g/l) 2 2                                                   15 H.sub.2 flow rate 7 150 230                                                18 discharge 1.4 3                                                             (m.sup.3 /h)                                                                 Reactor 6 volume (m.sup.3) 200 4500 4500                                      Reactor 36 volume (m.sup.3) n/a n/a 450                                       Reactor 17 volume (m.sup.3) 70 475 750                                      ______________________________________                                         *at 20 bar                                                               

EXAMPLE 2 Removal of sulphur compounds in the absence of significantamounts of nitrogen compounds.

Fuel gas containing H₂ and CO as main constituents and containing H₂ Sas the main pollutant is treated in an installation as shown in FIG. 1,in which, however, the trickling filter 19 with associated facilities20-23 has been dispensed with.

The results of the treatment according to this example are shown in thetable below.

EXAMPLE 3 Removal of sulphur compounds and nitrogen compounds

Fuel gas (synthesis gas) containing H₂ and CO as the main constituentsand containing H₂ S, COS, NH₃ and HCN as the main pollutants is treatedin an installation as shown in FIG. 2.

The polluted gas passes via line 1 into the bottom of gas scrubber 31,where in the main NH₃ is washed out of the gas with wash liquid which issupplied via 33 from aerobic reactor 36. The partially treated gasleaves the scrubber at the top and is fed via line 32 to the gasscrubber 5, where the bulk of H₂ S and HCN is removed using wash liquid3. In order to remove COS and residual H₂ S and HCN, the gas is fed via2 to gas scrubber 30, where it is washed with wash liquid supplied via22. The wash liquid for scrubbers 5 and 30 originates from the aerobicreactor 6.

The loaded wash liquids 34, 4 and 23 leave the scrubbers at the bottomand are fed to the aerobic reactor 6. In the aerobic reactor 6 sulphidein the wash liquid is converted mainly to sulphur by bacteria andoxygen. Only a small portion of the ammonia is oxidised to nitrate here.The reactor is supplied with air via aeration system 7. The spent aircan be fed via 8 to the second aerobic reactor and re-used foroxidation.

A portion of the effluent from the aerobic reactor 6 is used as washliquid for the gas scrubbers 5 and 30. The remaining portion is fed via10 to the sulphur separator 11, where the bulk of the sulphur is removedtherefrom. The sulphur slurry separated off is discharged via 12,dehydrated and, if necessary, purified for re-use.

The effluent from the sulphur separator is fed via 13 to the secondaerobic reactor 36. With the aid of bacteria and oxygen, the residualammonia is converted to nitrate. The reactor is supplied with air via anaeration system. To this end, the off-gas 8 from the first aerobicreactor is supplemented with air stream 35. The spent air can usually bereleased via 37 into the outside air without any problem.

The effluent from the aerobic reactor 36 is tapped off via 38 and aportion is used as wash liquid 33 for the gas scrubber 31. The remainingportion is fed via 39 to the anaerobic reactor 17. An electron donor isadded via 15. In the case of a gaseous electron donor, a discharge gasis released via 16 and, optionally, a portion of this gas is recycled.In the anaerobic reactor 17 sulphate is converted to sulphide.

The effluent is returned via 14 to the aerobic reactor 6, where thesulphide is again mainly converted to sulphur. A small discharge 18 isrequired to prevent accumulation of pollutant, non-degradablecomponents. Said discharge must be after-treated if necessary.

The results of the treatment according to this example are shown in thetable above.

I claim:
 1. Process for the treatment of a gas containing hydrogensulphide, the gas being washed in a first gas scrubber (5) with analkaline wash liquid and the spent wash liquid being treated in a firstaerobic reactor (6) with oxygen in the presence of sulphide-oxidisingbacteria and the effluent from the first aerobic reactor (6) beingre-used as wash liquid and elementary sulphur formed during thetreatment with oxygen being removed from the effluent, characterised inthat the effluent from which sulphur has been separated off is treatedin an anaerobic reactor (17) with sulphate-reducing bacteria andreturned to the first aerobic reactor (5).
 2. Process according to claim1, wherein the wash liquid in the first gas scrubber (5) has a pH of8-9.5.
 3. Process according to claim 1 wherein an electron donor such ashydrogen or a readily oxidisable organic substance is added to theanaerobic reactor (17).
 4. Process according to claim 1 wherein the gascontaining hydrogen sulphide also contains sulphur dioxide.
 5. Processaccording to claim 1 wherein the gas containing hydrogen sulphide alsocontains ammonia and the gas is washed in a second gas scrubber (31)with a wash liquid and the spent wash liquid from the second gasscrubber (31) is treated together with the spent wash liquid from thefirst gas scrubber (5).
 6. Process according to claim 5, wherein the gasis first washed in the second gas scrubber (31) and the wash liquid inthe second gas scrubber has a pH of 6-8.5.
 7. Process according to claim5 wherein the spent wash liquid from which sulphur has been separatedoff is treated with nitrifying bacteria in a second aerobic reactor (36)prior to the treatment in the anaerobic reactor.
 8. Process according toclaim 7, wherein the effluent from the second aerobic reactor (36) isused as wash liquid in the second gas scrubber (31).
 9. Processaccording to claim 1 wherein the gas also contains hydrocyanic acid andthe alkaline wash liquid is also provided with 1-50 g elementary sulphurper
 1. 10. Process according to claim 1 wherein the gas also containscarbonyl sulphide and the first gas scrubber (5) is constructed induplicate (5, 19/30).
 11. Process according to claim 1 wherein the gascontains less than 5% carbon dioxide, and carbon dioxide, (bi)carbonateor an organic substance which can be reacted to form (bi)carbonate isadded to the wash liquid.
 12. Installation for carrying out the processaccording to claim 1, comprising at least one gas scrubber with feed anddischarge lines for gas and means for the supply, distribution,collection and removal of wash liquid, said at least one gas scrubberbeing connected downstream through liquid lines with a first of at leastone aerobic reactor with gas feed lines and gas discharge lines andmeans for the supply and removal of liquid, the first aerobic reactorbeing connected downstream with a separator for separating solid fromliquid, and the separator being connected downstream with an anaerobicreactor having a feed line for an electron donor and means for thesupply and removal of liquid.